Interleaved Acousto-Optical Device Scanning For Suppression Of Optical Crosstalk

ABSTRACT

A method of scanning a sample includes simultaneously forming a plurality of co-linear scans. Each scan is formed by a sweep of a spot by an acousto-optical device (AOD). The co-linear scans are separated by a predetermined spacing. A first plurality of swaths are formed by repeating the simultaneous forming of the plurality of co-linear scans in a direction perpendicular to the co-linear scans. The first plurality of swaths have an inter-swath spacing that is the same as the predetermined spacing. The predetermined spacing can be a scan length or an integral number of scan lengths. A second plurality of swaths can be formed adjacent to the first plurality of swaths. Forming the second plurality of swaths can be performed in an opposite direction to that of the first plurality of swaths or in a same direction. An inspection system can implement this method by including a diffractive optical element (DOE) path after a magnification changer.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/844,576, entitled “INTERLEAVED ACOUSTO-OPTICAL DEVICE SCANNING FORSUPPRESSION OF OPTICAL CROSSTALK” filed Mar. 15, 2013.

BACKGROUND OF THE SPECIFICATION

1. Field of the Invention

Acousto-optical device scanning techniques and systems that suppressoptical crosstalk are described.

2. Related Art

During semiconductor fabrication, isolated and/or systemic defects maybe formed on the wafer. Isolated defects, which are present in a lowpercentage of chips on the wafer, may be caused by random events such asan increase in particulate contamination in a manufacturing environmentor an increase in contamination in the process chemicals used in thefabrication of the chips. Systemic defects, which are typically presentin a high percentage of chips on the wafer, may be caused by defects ona reticle. A reticle is used to transfer a pattern for an integratedcircuit layer onto the wafer using photolithographic techniques.Therefore, any defect on the reticle may be transferred with the patternto each chip of the wafer.

Automated inspection systems have been developed to inspect a wafersurface (both unpatterned and patterned). An inspection system typicallyincludes an illumination system and a detection system. The illuminationsystem may include a light source (e.g. a laser) for generating a beamof light and an apparatus for focusing and scanning the beam of light.Defects present on the wafer surface may scatter the incident lightprovided by the illumination system (also called an illuminator). Thedetection system is configured to detect the scattered light and convertthe detected light into electrical signals that can be measured,counted, and displayed. The detected signals may be analyzed by acomputer program to locate and identify defects on the wafer. Exemplaryinspection systems are described in U.S. Pat. No. 4,391,524, issued toSteigmeier et al. on Jul. 5, 1983, U.S. Pat. No. 4,441,124, issued toHeebner et al. on Apr. 3, 1984, U.S. Pat. No. 4,614,427, issued toKoizumi et al. on Sep. 30, 1986), U.S. Pat. No. 4,889,998, issued toHayano et al. on Dec. 26, 1989, and U.S. Pat. No. 5,317,380, issued toAllemand on May 31, 1994, all of which are incorporated by referenceherein.

One or more components used in a state-of-the-art illumination systemmay use acousto-optics. For example, FIG. 1A illustrates a simplifiedconfiguration of an acousto-optical device (AOD) 100. AOD 100 includes asound transducer 121, a quartz plate 122, and an acoustic absorber 123.An oscillating electric signal can drive sound transducer 121 and causesit to vibrate. In turn, this vibration creates sound waves in quartzplate 122. Acoustic absorber 123 is configured to absorb any sound wavesthat reach the edge of quartz plate 122. As a result of the sound waves,incoming light 124 to quartz plate 122 is diffracted into a plurality ofdirections 128, 129 and 130.

A diffracted beam emerges from quartz plate 122 at an angle that dependson the wavelength of the light relative to the wavelength of the sound.By ramping frequencies from high to low, portion 126 may have a higherfrequency than portion 127. Because portion 126 has a higher frequency,it diffracts a portion of the incident light beam through a steeperangle as shown by diffracted beam 128. Because portion 127 has arelatively lower frequency, it diffracts a portion of the incident lightbeam through a more shallow angle as shown by diffracted light beam 130.Because a mid-section portion between portions 126 and 127 has afrequency between the higher and relatively lower frequencies, itdiffracts a portion of the incident light beam through an intermediateangle as shown by diffracted light beam 129. This is an example of howan AOD can be used to focus an incoming beam 124 at position 125.

Notably, AODs can operate significantly faster than mechanical devices,such as mirrors. Specifically, AODs can diffract incoming light inapproximately the time it takes the sound wave to cross the incominglight beam (e.g. 5-100 ns). Thus, a scan of a sample, e.g. of a wafer orreticle, can be performed at a rate of, for example, 6.32 mm/μsec.

FIG. 1B illustrates an exemplary dual AOD illumination system 110configured to generate and scan a beam across a sample 109, such as awafer. A prescan AOD 101 is used to deflect the incident light from alight source 100 at an angle, wherein the angle is proportional to thefrequency of the radio frequency (RF) drive source. A telephoto lens 102is used to convert the angular scan from prescan AOD 101 into a linearscan.

A chirp AOD 104 is used to focus the incident beam in the plane ofacoustic propagation onto a scan plane 105. This is accomplished byramping thru all the RF frequencies with transducer 104A faster thanthose frequencies can all propagate thru chirp AOD 104. This rapidramping forms a chirp packet 104B. Chirp packet 104B then propagatesthru chirp AOD 104 at the speed of sound. FIG. 1B shows the location ofchirp packet 104B at the start of a spot sweep, whereas FIG. 1Cillustrates the location of chip packet 104B at the end of that spotsweep. Note that during this propagation, prescan AOD 101 adjusts its RFfrequency to track the chirp packet in AOD 104 to keep the light beamincident upon chirp packet 104B.

A cylinder lens 103 is used to focus the beam in a plane perpendicularto the plane of acoustic propagation. A relay lens 106 is used togenerate a real pupil at a pupil plane 106A. A magnification changer 107is used to adjust the size of the spot and the length of sweep. Anobjective lens 108 is used to focus the spot onto a sample 109, such asa wafer.

FIG. 2 illustrates another exemplary illumination system 200 using asingle AOD. In system 200, the prescan AOD is replaced by a beamexpander 201. Therefore, this type of illumination system is called a“flood AOD” system. In this configuration, multiple chirp packets 203Aand 203B are generated in AOD 104. Note that components having the samenumerical references herein are substantially similar components andtherefore their descriptions are not repeated. Each chirp packet 203Aand 203B generates its own spot. Therefore, objective lens 108 focusestwo spots onto sample 109 simultaneously. Although two chip packets areshown in FIG. 2, in other embodiments, additional chirp packets may begenerated with a corresponding number of spots incident on sample 109.

Note that sample 109 is typically placed on an XY translation stagecapable of bi-directional movement. In this configuration, the stage canbe moved so that the focused spots (formed by the focusing optics usingthe diffracted light beams) impinging sample 109 can be scanned alongadjacent contiguous strips of equal width (i.e. raster scan lines). U.S.Pat. No. 4,912,487, issued to Porter et al. on Mar. 27, 1990, andincorporated by reference herein, describes exemplary illuminationsystems including a translation stage configured to provide rasterscanning.

FIG. 3 illustrates a known exemplary AOD scanning technique providingisolation of scattered light for multiple spots. In this embodiment,four spots are scanned during four times 301, 302, 303, and 304 (eachspot having a same fill pattern for ease of reference in FIG. 3). Thesefour spots can be generated by an illumination system including an AOD.In FIG. 3, the AOD provides a chirp packet spacing 306 (which alsocorrelates to the spot spacing and the scan line segment length).

FIG. 4 illustrates an exemplary inspection system 400 for the techniquedescribed in FIG. 3. In system 400, an AOD optical path, e.g. similar tothat shown in FIG. 2, can include an objective lens 404 for focusing thespots generated by the AOD onto a sample 401. System 400 furtherincludes a 50/50 beam splitter (or other ratio) 405 that can direct twocopies of the scattered light 402 from the scanned spots on sample 401to two detector arrays 408 and 409. A first collection path and mask set406 can be configured to isolate the scattered light from a first set ofspots and provide its output to detector array 408, whereas a secondcollection path and mask set 407 can be configured to isolate thescattered light from a second set of spots and provide its output todetector array 408. Note that each mask has a set of windows, eachwindow having a predetermined width for a given PMT (photomultipliertube) or other sensor.

Referring back to FIG. 3, the first set of spots is indicated by theboxes having solid lines, whereas a second set of spots is indicated bythe boxes having dotted lines. The length of the boxes corresponds to awindow width 305 used for the masks in FIG. 4. Thus, for example, attime 301, the scattered light from spots 310 and 312 (using collectionpath and mask set 406) can be isolated from spot 311 (using collectionpath and mask set 407). To ensure complete coverage, a mask overlap 307is provided.

In the scanning technique of FIG. 3, two requirements must be met.First, PMT window width 305 must be smaller than the desired linesegment length, which is the spacing of the AOD chirp packet as shown by306. Second, the PMT windows must overlap, as shown by overlap 307, butmust not extend beyond the desired segment length. This requirementensures that only one spot is within a given mask at any time. Assumingboth requirements are met, the scanning technique of FIG. 3 can provideappropriate isolation for the scattered light because at no time arethere two spots in a single box.

However, this mask overlap can sometimes result in both arrays ofdetectors capturing the scattered light from the same spot, as shown byspot 313 at time 301. A similar condition occurs during time 304 forspot 314. This duplicated information must be recognized and accountedfor during analysis, thereby increasing collection system complexity.Note also that sometimes a spot is not within the area designated for amask, as shown by area 315 for time 302 and area 316 for time 303. Inthose cases, information must still be captured even though no spot ispresent, thereby wasting resources.

Moreover, 50/50 beam splitter 405 undesirably reduces the lightavailable for detection by one-half. To overcome this disadvantage, alaser (light source) that is 2× higher power would be needed, therebyincreasing the cost of the inspection system. Assuming the maximum powerlaser is already being used, an inspection system using a 50/50 beamsplitter would require a large laser. Having multiple chirp packets inthe AOD simultaneously as shown in FIG. 2 would have high spot-to-spotcrosstalk because of the relatively close proximity of the spots to oneanother. Moreover, because the PMT window is smaller than the desiredline segment length more PMTs are needed, thereby yet further increasinginspection system cost.

FIG. 5A illustrates another exemplary AOD illumination system 500 thatcan generate multiple spots without flood illumination. In thisembodiment, a diffractive optical element (DOE) 501 can be positionedbefore magnifier changer 107 to generate a plurality of spots. AlthoughFIG. 5A shows three spots being generated (different line colorsindicating different beams associated with those spots), otherembodiments can generate a different number of spots. FIG. 5Billustrates the effects of changing the magnification of magnifierchanger 107 on the spot size, spot spacing, and scan length on sample109 for illumination system 500. Note that the different fill colorsindicate different spots (and correspond to the different line colors ofFIG. 5A). As shown in FIG. 5B, large spots 520 have spacing associatedwith three positions 1, 3, and 5, whereas small spots 521 have spacingassociated with three positions 2, 3, and 4. The large spot in position1 scans to position 3, the large spot in position 3 scans to position 5,and the large spot in position 5 scans to position 7. In contrast, thesmall spot in position 2 scans to position 3, the small spot in position3 scans to position 4, and the small spot in position 4 scans toposition 5.

Having a smaller spot size (higher magnification), makes appropriateisolation for the scattered light from the multiple spots moredifficult. For example, FIGS. 6A and 6B illustrate exemplary sweeps ofthree small spots 601, 602, and 603 (corresponding to those shown inFIG. 5B) between times T₁ and T₄. FIG. 6B represents the scans of spots601, 602, and 603 as boxes of the same color, wherein the boxesrepresent the paths of the spots as a result of the propagation throughthe chirp AOD. FIG. 6B shows that there is an overlap of the co-linearscans of different spots (which would occur for both the big spots andthe small spots). This overlap will result in undesirable spotcrosstalk.

To provide the appropriate isolation between spots, thereby minimizingcrosstalk, additional optics and techniques are required. In oneembodiment, shown in FIGS. 7A and 7B, a prism 705 can be used in anillumination system to create the appropriate spacing between the spots.U.S. Pat. No. 7,075,638, issued to Kvamme on Jul. 11, 2006, andincorporated by reference herein, describes such an illumination system.In this system, prism 705 and additional optics, such as a sphericalaberration correction lens and a transmitted lens, are positioned suchthat scattered light from the plurality of spots, e.g. beams associatedwith spots 701, 702, and 703, on the sample are directed to a specificfacet of prism 705, as shown in FIG. 7A. In turn, prism 705 directs eachbeam to a separate detector. FIG. 7B shows the scan sweeps of spots 701,702, and 703 during operation of the associated inspection system. Prism705 (which is part of the collector) takes advantage of an offset shownin FIGS. 7A and 7B (the offset being generated by a grating, which ispart of the illumination system) to desirably increase the spotisolation. Thus, referring back to FIG. 6B, turning a grating willresult in spots 701, 702, and 703 (and their associated scans) no longerbeing co-linear along the x-axis (i.e. they will instead form a diagonalline with offset scans in a horizontal plane). Unfortunately, prism 705is designed for a specific magnification. Therefore, if themagnification is changed, then another prism must be used, therebyadding cost and design complexity to the inspection system.

The accurate detection of defects on a sample surface depends on thecorrect measurement and analysis of each spot in the scan. Therefore, aneed arises for optimizing techniques and systems using AODs that ensurethe isolation of these spots, thereby minimizing crosstalk, whileminimizing system complexity and cost.

SUMMARY

A method of scanning a sample is described. In this method, a pluralityof co-linear scans are simultaneously formed. Each scan is formed by asweep of a spot by an acousto-optical device (AOD). The co-linear scansare separated by a predetermined spacing. A first plurality of swathsare formed by repeating the simultaneous forming of the plurality ofco-linear scans in a direction perpendicular to the co-linear scans. Thefirst plurality of swaths have an inter-swath spacing that is the sameas the predetermined spacing.

In one embodiment, the predetermined spacing is a scan length. Inanother embodiment, the predetermined spacing is an integral number ofscan lengths. In yet another embodiment, an AOD parameter can beadjusted to provide an integral number of scan lengths as thepredetermined spacing.

The method can further include forming a second plurality of swathsadjacent to the first plurality of swaths. In one embodiment, the secondplurality of swaths is adjacent to all of the first plurality of swathsexcept a bottom half of the first plurality of swaths. Forming thesecond plurality of swaths can be performed in an opposite direction tothat of the first plurality of swaths or in a same direction to that ofthe first plurality of swaths.

Another method of performing a scan of a sample is described. In thismethod, a spot size and a first scan length is provided using anadjustable magnification changer, a spot separation is provided by adiffractive optical element (DOE) path, and a second scan length isprovided by a programmable acousto-optical device (AOD) based on thefirst scan length. The scan can be performed using the spot size, thespot separation, and the second scan length.

An inspection system is also described. This inspection system includesfirst and second AODs, a lens, a magnification changer, a firstdiffractive optical element (DOE) path, and a moveable platform. Thefirst AOD is configured to receive a light beam from a laser and todirect the light beam at various angles along an angular scan. The lensis configured to convert the angular scan to a linear scan. The secondAOD is configured to receive the light beam in the linear scan and togenerate a scan, the scan being a sweep of a spot, thereby generating aplurality of co-linear spots. The magnification changer is configured toadjust the magnification of the plurality of co-linear spots, therebygenerating an adjusted plurality of co-linear spots. The first DOE pathis configured to duplicate the adjusted plurality of co-linear spots,thereby generating a set of co-linear scans having a predeterminedspacing there between. The moveable platform system is configured tosecure a sample and form a first plurality of swaths by moving in adirection perpendicular to the co-linear scans as the first DOE pathgenerates a plurality of sets of the co-linear scans. This movementforms adjacent sets of the co-linear scans. The first plurality ofswaths have an inter-swath spacing equal to the predetermined spacing.

The moving platform system is further configured to step in a directionparallel to the co-linear scans and, with the first DOE path, generate asecond plurality of swaths. In one embodiment, the second plurality ofswaths are formed adjacent to the first plurality of swaths. In anotherembodiment, the second plurality of swaths are formed adjacent to thefirst plurality of swaths except for a bottom half of the firstplurality of swaths.

The predetermined spacing may be a scan length, an integral number ofscan lengths, or a non-integral number of scan. In one embodiment, thesecond AOD is programmable to provide an adjustable scan length for thesecond plurality of swaths.

The second plurality of swaths can be formed in an opposite direction tothat of the first plurality of swaths or in a same direction to that ofthe first plurality of swaths.

The first DOE path is for either normal incidence illumination oroblique incidence illumination. In one embodiment, the inspection systemfurther includes a second DOE path and a switching component configuredto direct the plurality of co-linear spots to one of the first DOE pathand the second DOE path.

The inspection system can further include an anamorphic waist relaypositioned to receive the light beam from the laser and configured toallow making adjustments to two independent axes.

When the laser includes a barium borate laser doubling crystal, theinspection system can further include a beam shaper having a slit. Theinspection system can further include a pupil and one or moreapodization plates placed in operative relation to the pupil andconfigured to provide a predetermined transmission profile (e.g. in thex-axis and the y-axis) to the plurality of co-linear spots. In oneembodiment, the pupil is decentered with respect to objective lenses ofthe first DOE path. The inspection system may also include an angle ofincidence mirror positioned between the magnification changer and thefirst DOE path. The angle of incidence mirror can be configured toadjust an angle of incidence to the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a simplified configuration of an acousto-opticaldevice (AOD).

FIG. 1B illustrates an exemplary dual AOD illumination system configuredto generate and scan a beam across a sample, such as a wafer.

FIG. 1C illustrates the location of a chip packet at the end of a spotsweep of the dual AOD illumination system shown in FIG. 1B.

FIG. 2 illustrates another exemplary illumination system using a singleAOD.

FIG. 3 illustrates a known exemplary AOD scanning technique providingisolation of scattered light for multiple spots.

FIG. 4 illustrates an exemplary inspection system for the techniquedescribed in FIG. 3.

FIG. 5A illustrates another exemplary AOD illumination system that cangenerate multiple spots without flood illumination.

FIG. 5B illustrates the effects of changing the magnification of themagnifier changer on the spot size, spot spacing, and scan length on asample for the illumination system shown in FIG. 5A.

FIGS. 6A and 6B illustrate exemplary sweeps of three small spots.

FIGS. 7A and 7B illustrate how a prism can be used in conjunction withan illumination system to create an appropriate isolation of spots inthe collector optics.

FIG. 8A illustrates an improved dual AOD illumination system configuredto generate and scan multiple spots across a sample.

FIG. 8B illustrates the effects of changing the magnification of themagnifier changer on the spot size and spot spacing on the sample forthe illumination system shown in FIG. 8A.

FIGS. 9A and 9B illustrate exemplary scans of three large spots andthree small spots generated by the illumination system shown in FIG. 8A.FIG. 9C illustrates the superposition of the scans of the large andsmall spots shown in FIGS. 9A and 9B.

FIGS. 10A and 10B illustrate a spot size and scan size comparison forbig spots and small spots at various points in an illumination system.

FIGS. 11A-11D illustrate a technique for scanning using the small spotsof FIG. 10B.

FIGS. 12A, 12B, and 12C illustrate a spot size and scan size comparisonfor big spots and small spots at various points in another illuminationsystem.

FIGS. 13A-13D illustrate a technique for scanning using the small spotsof FIG. 12B.

FIG. 14 illustrates an exemplary inspection system that can isolate thescattered light from multiple spots.

FIGS. 15A and 15B illustrate how the angle of incidence can be changedin an oblique illumination system.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 8A illustrates an improved dual AOD illumination system 800configured to generate and scan multiple spots across a sample 810, suchas a wafer. A prescan AOD 801 is used to deflect the incident light froma light source 800 at an angle, wherein the angle is proportional to thefrequency of the radio frequency (RF) drive source. A telephoto lens 802is used to convert the angular scan from prescan AOD 801 into a linearscan.

A chirp AOD 804 is used to focus the incident beam in the plane ofacoustic propagation onto a scan plane 805. This is accomplished byramping thru all the RF frequencies with its transducer 804A faster thanthose frequencies can all propagate thru chirp AOD 804. This rapidramping forms a chirp packet 804B. Chirp packet 804B then propagatesthru chirp AOD 804 at the speed of sound. The location of chirp packet804B propagates across chirp AOD 804 during the spot sweep (see, e.g.FIGS. 1B and 1C for a similar movement). Note that during thispropagation, prescan AOD 801 adjusts its RF frequency to track the chirppacket in AOD 804 to keep the light beam incident upon chirp packet804B.

A cylinder lens 803 is used to focus the beam in a plane perpendicularto the plane of acoustic propagation. A relay lens 806 is used togenerate a real pupil at a pupil plane 806A. A magnification changer 807is used to adjust the size of the spot and the length of sweep. Notably,a diffractive optical element (DOE) 808 is positioned aftermagnification changer 807 and before an objective lens 809. DOE 808makes copies of the spot output by magnification changer 807 withoutchanging the spot spacing, as described below. Although FIG. 8A showsthree spots being generated by DOE 808, other embodiments may have adifferent number of spots. Objective lens 809 is used to simultaneouslyfocus the multiple spots onto sample 810.

FIG. 8B illustrates the effects of changing the magnification ofmagnifier changer 807 on the spot size and spot spacing on sample 810for illumination system 800. Note that the different fill colorsindicate different spots (and correspond to the different line colors ofFIG. 8A). As shown in FIG. 8B, both large spots 820 and small spots 821can have identical spacing associated with three positions 1, 3, and 5.

FIG. 9A illustrates exemplary scans of three large spots 901, 902, and903 (corresponding to spots 820 shown in FIG. 8B) between times T₁ andT₄. FIG. 9A represents the sweeps of spots 901, 902, and 903 as boxes911, 912, and 913, respectively. FIG. 9B illustrates exemplary scans ofthree small spots 921, 922, and 923 (corresponding to spots 821 shown inFIG. 8B) between times T₁ and T₄. FIG. 9B represents the scans of spots921, 922, and 923 as boxes 931, 932, and 933, respectively. FIG. 9Cshows that large spot 901 in position 1 scans to position 5, large spot902 in position 9 scans to position 13, and large spot 903 in position17 scans to position 21. In contrast, small spot 921 in position 2 scansto position 4, small spot 922 in position 10 scans to position 12, andsmall spot 923 in position 18 scans to position 20. Therefore, the scansof the three small spots can be “nested” in the scans of the three largespots. That is, scans 931, 932, 933 can be nested in scans 911, 912, and913, respectively. As shown in FIG. 9C, when the collection optics aredesigned to collect the light from the low magnification configuration(big spot), they will by default collect the light from the highmagnification configuration (small spot).

FIGS. 10A and 10B illustrate a spot size and scan size comparison forbig spots and small spots at various points in an illumination system.As shown, chip AOD 804 (see FIG. 8) generates spots having the same spotsize and scan size; however, the magnification changer 807 changes boththe spot size and scan size. In one embodiment, both big and small spotsare reduced in size; however, magnification changer 807 creates a sizedifferential between the big spots and the small spots at this stage.Note that the magnification also changes the image orientation (shown bythe spot position switch from one side to another in the scan), which istypical for a magnifier. DOE 808 creates copies of the spots, wherein asnoted above, the scan positions between big and small spots is the same,although the spacing between the scans for the big and small spotsdiffers. Specifically, there is a bigger spacing between the small spotscans than for the big spot scans.

FIGS. 11A-11D illustrate a technique for scanning using the small spotsof FIG. 10B. Notably, this technique can be used for both normalincidence illumination as well as oblique incidence illumination. Inthis embodiment, completed scans (two spots shown for simplicity) form adashed co-linear line. That is, referring to FIG. 11A, scans 1101 and1102 (formed bottom to top in this case) have a spacing S1 therebetween. Therefore, a large spatial separation of the beams is ensured.In one embodiment, the scans are formed vertically (as shown) and theswaths are formed horizontally. For example, referring to FIG. 11B,swaths 1111 and 1112 can be formed by repeating the scans 1101 and 1102,respectively in a left to right movement. After the swaths are formed(with corresponding spaces S there between, also called blanks herein),another set of swaths can be formed in the spaces/blanks in a directionopposite to that used to form the previous sets of swaths. For example,swaths 1120 and 1121 can be formed right to left (shown only as partialswaths for clarity), whereas swaths 1101 and 1102 (see arrows of FIG.11A, and resulting swaths in FIG. 11B,) were formed left to right. Inone embodiment, a stage that positions the wafer can be stepped, e.g. byone scanning beam, to form each new scan (i.e. column of spots). Afterthe swath interleaving is complete, additional interleaved swaths can beformed in a similar manner to that described in FIGS. 11A-11C to providea complete scan of a sample, as shown in FIG. 11D (which assumes a3-spot DOE).

Note that spacing S1 can be designed to fit an integral number of scans,thereby ensuring a complete scan of the wafer without duplicateinformation. In one embodiment, this inter-swath spacing can be adjustedusing a chirp parameter of a programmable AOD, which is described belowin reference to FIG. 14. Because spacing S1 is greater than one scanlength, a few small vertical adjustments (e.g. one scan length) can bemade to create the necessary fill-in swaths before a large verticaladjustment is made.

FIGS. 12A and 12B illustrate a spot size and scan size comparison forbig spots and small spots at various points in another illuminationsystem. In this embodiment, a programmable AOD 804A with softwarecontrol can be used. This software control allows a longer scan to beselectively generated, if desired. However, the functioning ofmagnification changer 807 and DOE 808 is the same as that describedabove. That is, chip AOD 804 (see FIG. 8) generates spots having thesame spot size and scan size; however, the magnification changer 807changes both the spot size and scan size. DOE 808 creates copies of thespots. Notably, by using programmable AOD 804A, the scan size (but notspot size) can be increased. Therefore, DOE 808 can generate scans withsmaller spacing there between. FIG. 12C shows that small spot 1201 inposition 1 scans to position 5, small spot 1202 in position 9 scans toposition 13, and small spot 1203 in position 17 scans to position 21.Notably, scans 1210 (including spot 1201), 1211 (including spot 1202),and 1212 (including spot 1203) have a spacing there between that permitsexactly one scan length. This spacing can be leveraged, as describedbelow.

Note that when comparing FIG. 10B with FIG. 12B, the throughput of aninspection system including the configuration of FIG. 12B will begreater. Specifically, the actual scan rate will be the same. The scanrate is proportional to the size of the scanning spot and the velocityof the scanning spot. In FIGS. 10B and 12 b, the size of the spot andthe velocity is the same. However, the XY stage will be slower for theconfiguration of FIG. 12B, i.e. the set-up is necessarily longer becauseit must still wait for the longer scan to finish. In addition, theheight of the swath is larger for FIG. 12B as indicated by the number ofpixels per segment shown in FIGS. 10A, 10B, 12A, and 12B, which in thisembodiment are all 650 pixels per scan, except for the configuration ofFIG. 12B, which is 1950 pixels per scan. Note that when a larger spotsize is provided, a correspondingly faster velocity results, and asmaller spot size corresponds to a slower velocity. Thus, it takes thesame amount of time for the large spot to travel its scan as the smallspot to travel its scan in FIGS. 10A and 10B. However, referring toFIGS. 12A and 12B, because the scan size is bigger (i.e. the scan lengthis longer), then it takes longer for the small spot to travel its scancompared to the large spot to travel its scan (1950 pixels versus 650pixels). Throughput is based on how fast a certain area of the wafer canbe scanned, which will be determined by the velocity of the scan and thespot size. Starting and stopping the XY stage for forming the swaths isoverhead because the spot scanning is not being done during thoseperiods. Therefore, reducing the overhead by reducing the number ofswaths and hence the number of times the XY stage must be stopped andstarted improves the throughput of the inspection system as illustratedby FIG. 12B. Moreover, it also takes time to set up the pre-scan AOD andother various electronics between scans. Thus, even more overhead can beaverted by lengthening the scan, e.g. as that shown in FIG. 12B. Thepeak data rate is always the rate for digitizing the pixels when thespot is moving. Thus, the peak data rate is the same for any ofconfigurations shown in FIGS. 10A, 10B, 12A, and 12B. However, theaverage data rate (which is always lower than the peak data rate) willvary based on the overhead. Thus, the configuration shown in FIG. 12Bprovides the fastest average data rate (and closest to the peak datarate) of the configurations shown in FIGS. 10A, 10B, 12A, and 12B.

FIGS. 13A-13D illustrate a technique for scanning using the small spotsof FIG. 12B. In this embodiment, completed scans (three spots shown forsimplicity) also form a dashed co-linear line with small spaces betweenscans. That is, referring to FIG. 13A, scans 1301 and 1302 (formedbottom to top in this case) have a spacing S2 there between. Spacing S2is smaller than spacing S1, however, sufficient spatial separation ofthe beams using spacing S2 is still ensured. The spacing S1 and spacingS2 are controlled by the length of the scan in the chirp AOD, and areoptimized when the spacing is equal to the length of the scan (scansize).

In one embodiment, the scans are formed vertically (as shown) and theswaths are formed horizontally. For example, referring to FIG. 13B,swaths 1310, 1311, and 1312 can be formed by repeating the scans 1301,1302, and 1303, respectively in a left to right movement. After theswaths are formed (with corresponding spaces S2 there between), anotherset of swaths can be formed in the spaces/blanks in a direction oppositeto that used to form the previous sets of swaths. For example, swaths1320, 1321, and 1322 can be formed right to left (shown only as partialswaths for clarity), whereas swaths 1310, 1311, and 1312 (see arrows ofFIG. 13A, and resulting swaths in FIG. 13B,) were formed in a left toright pattern. Once again, a stage that positions the sample can bestepped, e.g. by one scanning beam. After the swath interleaving iscomplete, additional interleaved swaths can be formed in a similarmanner to that described in FIGS. 13A-13C to provide a complete scan ofthe sample. Because spacing S2 is equal to one scan length, one smallvertical adjustment (i.e. one scan length or scanning beams) can be madeto create the necessary fill-in swath before a large vertical adjustmentis made. In one embodiment, shown in FIG. 13D, the stage can step 5scanning beams (e.g. 1⅔ of the field of view (FOV)). In anotherembodiment, and referring to FIG. 13B, instead of moving up one scanlength after finishing swaths 1310, 1311, and 1312, the next and allsubsequent vertical adjustments could be that shown by arrow 1350. Notethat this fill-in pattern depends on the number of spots, e.g. for a 5spot pattern, 2 blanks are left; for a 7 spot pattern, 3 blanks areleft. Thus, in general, a second plurality of swaths are formed adjacentto all of the first plurality of swaths except a bottom half of thefirst plurality of swaths. This fill-in pattern provides completecoverage with the exception of the space between swaths 1310 and 1311.In this configuration, swath 1311 would then designate the first area ofinterest on the wafer. This technique can be applied to otherconfigurations that utilize more spots.

FIG. 14 illustrates an exemplary inspection system 1400 that can isolatethe scattered light from multiple spots. Notably, system 1400 can useeither normal incidence illumination or oblique incidence illumination.As known by those skilled in the art, some defects are optimallyilluminated using normal incidence illumination and other defects areoptimally illuminated using oblique incidence illumination. Notably,multiple collectors 1430 a, 1430 b, 1430 c can collect the scatteredlight from a sample 1421, such as a wafer, using normal incidenceillumination or oblique incidence illumination without reconfiguration.Specifically, as described in further detail below, no magnificationchange is necessary in the collectors 1430A, 1430 b, and 1430 c becausethe illumination optics are configured as described in FIG. 9C to haveall spots overlap.

In inspection system 1400, light from a laser 1401 can be directed to ananamorphic waist relay (AWR) 1402. AWR 1402, which can includecylindrical lenses, prisms, gratings or spherical components (motorizedor non-motorized) provides the ability to make adjusts to spot size toaccount for variations in laser waist parameters and system fabricationand alignment tolerances. One preferred embodiment utilizes anamorphiccomponents that allow making adjustments to two independent axes. AWR1402 provides its output to a collimation lens 1403.

Collimation lens 1403 provides its output to a beam shaper 1405. Beamshaper 1405 is used to adjust the size of the beam at the entrance of aprescan AOD 1406. Moreover, if laser 1401 includes a laser BBO (bariumborate) doubling crystal, beam shaper 1405 can also include a slit tocondition the beam as a result of the BBO crystal. This slit in beamshaper 1405 can be implemented as a standard slit, or can include one ormore apodization plates or serrated slits to improve its function.

Beam shaper 1405 provides its output to prescan AOD 1406. Prescan AOD1406 is used in deflection mode and is used in conjunction withtelephoto lens 1407 and anamorphic beam expander 1408 to position andscan the beam in relation to a chirp AOD 1409. Prescan AOD 1406 scansthe laser beam through an angle. A lens 1407 converts the angular scanfrom prescan 1406 into a linearly translation scan. Lens 1407 can beimplemented as a telescope, beam expander, relay lens, focusing lens,objective lens, or any other appropriate optical component known in theart. An anamorphic beam expander 1408 is used to convert the circularoutput from prescan 1406 and telephoto 1407 into an oblong shape.Anamorphic beam expander 1408 can include cylindrical lenses, prisms,gratings, or spherical components. Note that the oblong-shaped beamprovided by anamorphic beam expander 1408 may be necessary toaccommodate limitations in the fabrication of chirp AOD 1409,specifically the height of the diffracting sound column.

Chirp AOD 1409 is used to focus the laser beam in the acousticpropagation direction and scan the laser beam. A transducer of chirp AOD1409 can be configured to generate a signal that produces a chirppacket, which propagates over a length of chirp AOD 1409 from a startposition to an end position. In one preferred embodiment, chirp AOD 1409and prescan AOD 1406 are programmable in software to improve the systemthroughput, as described above in FIGS. 12A and 12B.

The output of chirp AOD 1409 can be provided to components 1410, such asa cylinder lens, a relay lens, multiple field stops/slits andpolarization components. The cylinder lens is used to focus the scanningbeam in the axis perpendicular to the scanning motion of chirp AOD 1409.The relay lens is used to form a real pupil at the location ofdownstream components 1414 and 1413 (described below). The field stopsand slits are used to filter out unwanted diffraction orders from chirpAOD 1409 and pre-scan AOD 1406, as well as filter out any unwantedscattered light from other components (laser 1401 through components1410). In addition, the slits are used as field stops to accommodatechanges in the required line length. The polarization components caninclude components to both filter and generate a specific polarization,such as a Brewster plate polarizer, a wire grid polarizer, a prism, orany other components providing similar functionality. The polarizationcomponents can also include components to alter the polarization such asa half wave plate, quarter wave plate, or other plates providing similarfunctionality. These polarization components are used to providemultiple polarization options for inspecting the substrate.

Components 1410 provide their output to one or more apodization plates1413. Apodization plate 1413 is used to change the shape of the systemoptical point spread function in response to the challenges provided bythe sample being inspected. The apodization function can be accomplishedthrough the use of serrated sheet metal components, dot densitycomponents, coatings, or other methods known in the art. In oneembodiment, apodization plate 1413 can have independent control of thepoint spread function in the X and Y axes.

Apodization plate 1413 provides its output to a zero order filter slit1414. Slit 1414 is used to remove the zero order from the optical path.Zero order slit 1414 provides its output to a magnification changer1415. Magnification changer 1415 is used to adjust the overallillumination optics system magnification. In doing so, this changes thesize of the spot, spot velocity, and the length of scan at sample 1421.

An angle of incidence mirror 1416 is an adjustable mirror used to changethe angle of incidence to the wafer. When the system magnification issmall and the spot size is large, the resulting numerical aperture (NA)of the pupil is small. Therefore, angle of incidence mirror 1416 can beadjusted to increase the angle of incidence (from the wafer normal) forthe inspection beam. When the system magnification is large and the spotsize is small, the angle of incidence mirror is positioned to decreasethe angle of incidence (from the wafer normal) for the inspection beam.This adjustability provides benefits to filtering repeating structures,inspection speed, and defect signal to noise.

Angle of incidence mirror 1416 provides its output to a beam diverter1417. Beam diverter 1417 is used to select between the oblique incidenceillumination path, normal incident illumination path, or both obliqueand normal incidence paths.

In the oblique incidence path, beam diverter 1417 provides its output toa DOE 1418. DOE 1418 is used to make multiple copies of the scanningbeam as described previously.

DOE 1418 provides its output to an oblique fixed magnification 1419.Oblique fixed magnification 1419 is used to image the real pupil at theDOE 1418 location to the entrance pupil of an objective 1420. Objective1420 is used to focus the beam onto the substrate being inspected.

In the normal incidence path, beam diverter 1417 provides its output toa turning mirror 1425. Turning mirror 1425 provides its output to normalincidence anamorphic beam expander 1426. Anamorphic beam expander 1426can include cylindrical lenses, prisms, gratings or sphericalcomponents. Anamorphic beam expander 1426 is used to expand the beam inone axis or conversely reduce the beam in one axis. Thisexpansion/reduction flexibility provides benefits to filtering repeatingstructures, inspection speed, and defect signal to noise.

Anamorphic beam expander 1426 provides its output to a normal incidenceDOE 1427. Normal incidence DOE 1427 is used to make multiple copies ofthe scanning beam as described previously.

Normal incidence DOE 1427 provides its output to a normal incidencefixed magnification 1428. Normal incidence fixed magnification 1428 isused to image the real pupil at the location of DOE 1427 to the entrancepupil of an objective 1422. Objective 1422 is used to focus the spotonto the sample for the normal incidence channel.

Normal incidence fixed magnification 1428 provides its output through aturning mirror to a NI (normal incidence) beam shaper changer 1429. NIbeam shaper changer 1429 serves multiple functions. It has multipleplates that serve as apertures, mirrors and beam splitters. These beamsplitters can be configured to have multiple ratios for transmission andreflection (example 50/50, 100/0, 80/20, etc.). These beam splitters canalso have multiple transmission and reflection profiles in a spatialsense to enable various configurations of a collection channel 1430A.Collection of scattered light is not limited to light that passes thrunormal incidence objective 1422. Collection of light from the wafer canalso be achieved through additional collection channels 1430B and 1430C.

Sample 1421 can be secured by a moveable platform 1431. In oneembodiment, moveable platform 1431 can include a chuck, at least alinear motor (providing x-y movement), and a spindle motor (providingrotation) (optional). Moveable platform 1431 can be controlled by acentral control and data acquisition computer 1432 via a motor controlcable 1433. Note that moveable platform 1431 is moving perpendicular tothe direction of the scan (i.e. the sweep of the spot). In one preferredembodiment, moveable platform 1431 can be continuously moving becausethe scans are so much faster relative to the speed of the platform (e.g.on the order of μsecs for a scan versus seconds for the platform).Central control and data acquisition computer 1432 can receive inputsfrom collectors 1430A, 1430B, and 1430C.

As shown in FIGS. 15A and 15B, an angle of incidence adjuster 1501 isused in concert with the magnification changer provide multiple obliqueincidence angles to substrate 1504. FIG. 15A illustrates an optionalconfiguration for the low magnification, large spot, low NA (indicatedby 1502A) configuration. In this case the adjuster 1501 is moved (shownlowered closer to objective 1503) to provide for a higher angle ofincidence to substrate 1504. FIG. 15B shows the high magnification,small spot, high NA (indicated by 1502B) configuration with the adjuster1501 in its nominal position. This position can be used for allmagnification options.

As described above, the programmable chirp AOD and the DOE, which ispositioned after the magnification changer, form the scans in a firstdirection (in this case vertical), whereas moveable platform 1431 andcentral control and data acquisition computer 1432 form the swaths ofscans in a second direction (in this case, a horizontal direction),which is perpendicular to the first direction. The number of thescanning spots is equal to the number of swaths. An inspection systemincluding this configuration can provide flexible spacing betweenadjacent, co-linear scans to eliminate spot cross-talk. Moreover,because the DOE provides spacing between the spots, inexpensive,non-imaging collectors can be used.

The various embodiments of the structures and methods of this inventionthat are described above are illustrative only of the principles of thisinvention and are not intended to limit the scope of the invention tothe particular embodiments described. For example, although theembodiments are described with a predetermined number of spots, otherembodiments of an illumination system or an inspection system mayinclude a different number of spots. Thus, the invention is limited onlyby the following claims and their equivalents.

1. An inspection system for inspecting a sample, the inspection systemcomprising: a moveable platform system configured to secure said sample;an illumination system configured to simultaneously generate a pluralityof co-linear scans aligned along a co-linear scan line such that eachscan is formed by a sweep of a spot by an acousto-optical device (AOD)along the co-linear scan line, and such that the plurality of co-linearscans are directed onto the secured sample and separated by apredetermined spacing; and a controller configured to control themoveable platform system such that the secured sample is repeatedlystepped relative to the illumination system in a direction perpendicularto the co-linear scan line and in coordination with the generation ofthe plurality of co-linear scans such that a first plurality of swathsare formed by repeatedly generating the plurality of co-linear scans ina direction perpendicular to the co-linear scan line, the firstplurality of swaths having an inter-swath spacing of the predeterminedspacing.
 2. The inspection system of claim 1, wherein the illuminationsystem is further configured to simultaneously generate the plurality ofco-linear scans such that each scan has a scan length, and such that thepredetermined spacing is equal to the scan length.
 3. The inspectionsystem of claim 1, wherein the illumination system is further configuredto simultaneously generate the plurality of co-linear scans such thateach scan has a scan length, and such that the predetermined spacing isequal to an integral number of said scan lengths.
 4. The inspectionsystem of claim 3, wherein the AOD is programmable and configured suchthat said integral number of scan lengths is adjustable by adjusting achirp of said programmable AOD.
 5. The inspection system of claim 1,wherein said controller is further configured to control the moveableplatform system such that the plurality of co-linear scans form a secondplurality of swaths adjacent to the first plurality of swaths.
 6. Theinspection system of claim 1, wherein said controller is furtherconfigured to control the moveable platform system such that theplurality of co-linear scans form a second plurality of swaths adjacentto all of the first plurality of swaths except a bottom half of thefirst plurality of swaths.
 7. The inspection system of claim 6, whereinsaid controller is further configured to control the moveable platformsystem such that the second plurality of swaths are formed by moving thesample in an opposite direction to that utilizing during formation ofthe first plurality of swaths.
 8. The inspection system of claim 6,wherein said controller is further configured to control the moveableplatform system such that the second plurality of swaths are formed bymoving the sample in a same direction to that utilizing during formationof the first plurality of swaths.
 9. A method comprising: receiving alight beam from a laser and directing the light beam at various anglesalong an angular scan; converting the angular scan to a linear scan;receiving the light beam in the linear scan and generating a scan, thescan being a sweep of a spot, thereby generating a plurality ofco-linear spots; adjusting a magnification of the plurality of co-linearspots, thereby generating an adjusted plurality of co-linear spots;duplicating the adjusted plurality of co-linear spots such that eachadjacent pair of said co-linear spots are separated by a predeterminedspot spacing, thereby simultaneously generating a set of co-linear scansaligned along a co-linear scan line and having a predetermined scanspacing there between; and forming a first plurality of swaths by movinga sample in a direction perpendicular to the co-linear scan line, saidmoving said sample causing said plurality of sets of the co-linear scansto form adjacent sets of the co-linear scans, the first plurality ofswaths having an inter-swath spacing equal to the predetermined scanspacing, whereby performing said duplicating after said adjustingfacilitates controlling a size of said plurality of co-linear spotswithout changing said predetermined spot spacing between each adjacentpair of said co-linear spots.
 10. The method of claim 9, wherein movingthe sample comprises stepping the sample in a direction parallel to theset of co-linear scans and then moving the sample in a directionperpendicular to the co-linear scan line such that the sets of co-linearscans form a second plurality of swaths adjacent to the first pluralityof swaths.
 11. The method of claim 10, wherein simultaneously generatingsaid set of co-linear scans comprises sweeping said co-linear spots suchthat said first and second pluralities of swaths have a scan length, andsuch that the predetermined scan spacing is an integral number of saidscan lengths.
 12. The method of claim 11, wherein stepping the samplecomprises moving the sample an adjustment distance in the directionparallel to the set of co-linear scans such that the second plurality ofswaths are formed adjacent to the first plurality of swaths except for abottom half of the first plurality of swaths.
 13. The method of claim10, simultaneously generating said set of co-linear scans comprisessweeping said co-linear spots such that said first plurality of swathshave a first scan length and the second plurality of swaths have asecond scan length that is different from the first scan length.
 14. Themethod of claim 10, wherein forming the second plurality of swathscomprises moving the sample in an opposite direction to that utilizedduring forming the first plurality of swaths.
 15. The method of claim10, wherein forming the second plurality of swaths comprises moving thesample in a same direction to that utilized during forming the firstplurality of swaths.
 16. The method of claim 9, wherein duplicating theadjusted plurality of co-linear spots comprises diverting said adjustedplurality of co-linear spots either to normal incidence illuminationpath or an oblique incidence illumination path.
 17. The method of claim9, wherein duplicating the adjusted plurality of co-linear spotscomprises diverting said adjusted plurality of co-linear spots to anoblique incidence illumination path.
 18. The method of claim 9, whereinduplicating the adjusted plurality of co-linear spots comprisescontrolling a switching component7 to direct the adjusted plurality ofco-linear spots to one of a normal incidence illumination path and anoblique incidence illumination path.
 19. The method of claim 9, furthercomprising making adjustments to two independent axes of the light beamreceived from the laser before directing the light beam at said variousangles.
 20. The method of claim 9, further comprising generating thelight beam using a barium borate laser doubling crystal, and passing thelight beam through a beam shaper having a slit.
 21. The method of claim9, further including utilizing a pupil and one or more apodizationplates placed in operative relation to the pupil to provide apredetermined transmission profile to the plurality of co-linear spots.22. The method of claim 21, wherein utilizing the one or moreapodization plates includes configuring the one or more apodizationplates to provide a same transmission profile in an x axis and a y axis.23. The method of claim 21, wherein utilizing the one or moreapodization plates includes configuring the one or more apodizationplates to provide a different transmission profile in an x axis and a yaxis.
 24. The method of claim 21, wherein utilizing the one or moreapodization plates includes configuring the one or more apodizationplates to provide a programmable transmission profile.
 25. The method ofclaim 21, wherein utilizing the pupil includes decentering the pupilwith respect to objective lenses of the first DOE path.
 26. The methodof claim 9, further including utilizing an angle of incidence mirror toadjust an angle of incidence to the sample.